Printing plates are well known for use in flexographic printing, particularly on surfaces which are corrugated or smooth, such as packaging materials like cardboard, plastic films, etc. Typically, flexographic printing plates are manufactured using photopolymers which are exposed through a negative, processed using a solvent to remove the non-crosslinked areas to create a relief, which is post-crosslinked and detackified. This is typically a very lengthy and involved process. Recently, flexographic plates have been manufactured using digital imaging of an in situ mask layer which obviates the need for a negative or a photomask to make the plate, and which has other performance benefits as well.
Recently, it has been possible to laser engrave a rubber element directly to provide the desired relief surface necessary for flexographic printing. Laser engraving has provided a wide variety of opportunities for rubber printing plates. Highly concentrated and controllable energy lasers can engrave very fine details in rubber. The relief of the printing plate can be varied in many ways. Very steep as well as gently decreasing relief slopes can be engraved so as to influence the dot gain of such plates. Ethylene propylene diene monomer (EPDM) rubber can be laser engraved to form flexographic printing plates.
The directly engraved type of flexographic printing plate is made from vulcanized rubber. Commercial rubbers can be natural or synthetic, such as EPDM elastomers. Lasers can develop sufficient power densities to ablate certain materials. For example, high-power carbon dioxide (CO2) lasers can ablate many materials such as wood, plastic and rubber and even metals and ceramics. Once the output from a laser is focused at a particular point on a substrate with a suitable power density, it is possible to remove material to a desired depth to create a relief. Areas not struck by the laser beam are not removed. Thus, the use of the laser offers the potential of producing very intricate engravings in a desired material with substantial savings.
U.S. Pat. No. 3,459,733 to Caddell describes a method for producing polymer printing plates. The printing plate is made by exposing a layer of the polymeric material to a controlled laser beam of sufficient intensity to ablate the polymer and form depressions on the surface.
U.S. Pat. Nos. 5,798,202 and 5,804,353 to Cushner et al. disclose processes for making a flexographic printing plate by laser engraving a reinforced elastomeric layer on a flexible support. The process disclosed in U.S. Pat. No. 5,798,202 involves first reinforcing and then laser engraving a single-layer flexographic printing element having a reinforced elastomeric layer on a flexible support. The elastomeric layer may be reinforced mechanically, thermochemically, photochemically or with combinations of these processes. Mechanical reinforcement is provided by incorporating reinforcing agents, such as finely divided particulate material, into the elastomeric layer. Photochemical reinforcement is accomplished by incorporating photohardenable materials into the elastomeric layer and exposing the layer to actinic radiation. Photohardenable materials include photo-crosslinkable and photo-polymerizable systems having a photo-initiator or photo-initiator system.
The process disclosed in U.S. Pat. No. 5,804,353 is similar to U.S. Pat. No. 5,798,202, except that the process involves reinforcing and laser engraving a multilayer flexographic printing element having a reinforced elastomeric top layer, and an intermediate elastomeric layer on a flexible support. The elastomeric layer is reinforced mechanically, thermochemically, photochemically or combinations thereof. Mechanical and photochemical reinforcement is accomplished in the same manner as described by U.S. Pat. No. 5,798,202. The intermediate elastomeric layer may be reinforced as well.
A problem associated with elastomeric elements that are reinforced both mechanically and photochemically is that laser engraving does not efficiently remove the elastomeric material to provide desired relief quality, and ultimately, printing quality. It is desirable to use an additive in the elastomeric layer that is sensitive to infrared light in order to enhance the engraving efficiency of the element. Photo-chemically reinforcing the element provides the desired properties for engraving as well as in its end-use as a printing plate. However, the presence of the additive as particulate or other absorbing material tends to reduce the penetration of the ultraviolet radiation required to photo-chemically reinforce the element. If the elastomeric layer is insufficiently crosslinked during photochemical reinforcement, the laser radiation cannot effectively remove the material and poor relief quality of the engraved area results. Further, the debris resulting from laser engraving tends to be tacky and difficult to completely remove from the engraved element. Additionally, if the element is not sufficiently photo-chemically reinforced, the required end-use properties as a printing plate are not properly achieved. These problems tend to be exacerbated with increasing concentration of the additive that enhances engraving efficacy.
U.S. Pat. No. 6,627,385 teaches the use of graft copolymers for laser engraving. U.S. Pat. No. 6,511,784, U.S. Pat. No. 6,737,216 and U.S. Pat. No. 6,935,236 teach the use of elastomeric copolymers for laser engraving using various infrared (IR) additives.
Many patents in the field teach the use of typical styrenic thermoplastic elastomers (TPEs) that have been used for photo-crosslinking applications. One problem associated with these non-polar TPEs is that they have limited sensitivity to laser engraving because of their hydrocarbon backbone nature. The use of polar TPEs such as thermoplastic polyurethanes (TPUs) thermoplastic polyester elastomers (TPPE) and thermoplastic polyamide elastomers (TPAE) as both laser engravable systems and as printing elements would be desirable. However, most of the above polar TPEs on the market would not be effective either as laser engravable systems, or as printing plates because they are not crosslinked.
The crosslinking of the above TPEs and especially TPUs has not been done before in flexography, and thus, TPUs have not been used in flexography. However, polyurethanes for flexography have been well known, particularly for liquid photopolymers. By definition, a TPU is solid at room temperature and can be extruded, and is workable at higher temperatures. This characteristic is due to the presence of hard and soft segments that form a network at room temperature, and is thus solid.
This network structure also differentiates TPUs from traditional polyurethanes in its outstanding physical attributes and thus offers an attractive system to be used in flexo applications. However, most elastomers used in Flexo need to be crosslinked to withstand the rigors of the printing process and to minimize swells in the inks used for printing. Additionally, the elastomers used in laser engraving have to be crosslinked. Traditional flexo photopolymers have unsaturation in the backbone, which allows the crosslinking with acrylate monomers and UV photo-initiators. The TPUs on the market today do not have unsaturation. Hence, the difficulty in UV crosslinking these for flexo applications. Additionally, laser engraving of elastomers with lasers lasing in the Near IR wavelengths need to be doped with highly absorptive laser additives. This does not allow UV crosslinking as a viable option to crosslink such elastomers. Thermal crosslinking or vulcanization is the only feasible approach in such applications. Millable Polyurethanes (MPUs) are a special category of TPUs. Millable Polyurethanes, as the name suggests, could be processed in the same way as rubber elastomers, including the use of compounding and extrusion methods. MPUs can be thermally crosslinked in a subsequent crosslink and post-crosslink step.